public class DerivedObservableCollection<TBase, TValue> : ICollection<TValue>, System.Collections.Specialized.INotifyCollectionChanged
{
    private ICollection<TBase> baseCollection;
    private Func<TBase, TValue> selector;

    public event System.Collections.Specialized.NotifyCollectionChangedEventHandler CollectionChanged;

    public DerivedObservableCollection(ICollection<TBase> baseCollection, Func<TBase, TValue> selector)
    {
        this.baseCollection = baseCollection;
        this.selector = selector;

        var notifyCollectionChanged = (System.Collections.Specialized.INotifyCollectionChanged)this.baseCollection;
        notifyCollectionChanged.CollectionChanged += baseCollection_CollectionChanged;
    }

    void baseCollection_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        OnCollectionChanged(e);
    }

    protected void OnCollectionChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        // Convert 'e' so it represents TValue instead of TBase here.

        if (CollectionChanged != null)
            CollectionChanged(this, e);
    }

    #region ICollection<TValue> Members

    public void Add(TValue item)
    {
        throw new NotSupportedException();
    }

    public void Clear()
    {
        throw new NotSupportedException();
    }

    public bool Contains(TValue item)
    {
        return this.baseCollection.Any(i => this.selector(i).Equals(item));
    }

    public void CopyTo(TValue[] array, int arrayIndex)
    {
        throw new NotSupportedException();
    }

    public int Count
    {
        get { return this.baseCollection.Count; }
    }

    public bool IsReadOnly
    {
        get { return true; }
    }

    public bool Remove(TValue item)
    {
        throw new NotSupportedException();
    }

    #endregion

    #region IEnumerable<T> Members

    public IEnumerator<TValue> GetEnumerator()
    {
        return this.baseCollection.Select(i => this.selector(i)).GetEnumerator();
    }

    #endregion

    #region IEnumerable Members

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return this.baseCollection.Select(i => this.selector(i)).GetEnumerator();
    }

    #endregion
}